So I've thought a lot about it, and put a lot of details in that will probably never be brought up in RP, so I thought I'd post it here to help you guys understand.

Sae has what's known as MPD (Multiple Personality Disorder), though there's a new PC term for it which I don't remember at the moment. MPD is generally caused by a severely traumatizing event in which the only way for the mind to cope with the event, would be to create a separate persona, or alter to hold the memory for them.

There are several terms that I'll use and I'll describe them here.

The Core: The core persona, the main person from whom any new alters are made. Alters can only be formed from the Core. This is Sae.

The Host: The persona who generally has default control of the physical body. This is generally Sae, however at this point in time (5/14/11) this is Senka.

Alter: Another term for a separate persona. Saerun has three alters: Sae, Sobe and Senka.

Mindscape: The place where the alters reside when not in control. Each alter has their own "room" in the mindscape. Sae's room is the base room (representing her status as the Core) and is in the form of a living room.

Sobe's room is behind a veil in Sae's room. Her room is dark and is a labyrinth, made of crumbling stone.

Senka's room is behind a fissure in the wall of Sae's room. Her room is a garden gone completely wild.

The Power Play:

Sae is the Core, and hence is generally considered to be the "boss", as it is her mind. Therefore Sobe is forced to listen and obey if Sae gives her a direct command.

Control is taken by standing on the pedestal in the mindscape, which merges that consciousness into the body.

About Alters: The purpose of an alter is to hold memories that the Core is not ready to remember, and generally to aid the Core. An Alter cannot die unless killed/merged directly with the Core.

Senka: Senka is not a true alter in that she doesn't hold any memories to help Sae. Senka is what Sae *could* be when she merges with Sobe. However, her base personality has been corrupted by the destructive effects of Blue Magic. Because she is not a true alter, she does not listen to any orders given by Sae, and instead takes revenge by mentally torturing Sae and Sobe.

A crude picture of the Mindscape: